Unraveling the complexity of the mammalian brain is one of the most challenging problems in biology today. A major goal of neuroscience is to understand how circuits of neurons and non-neuronal cells process sensory information, generate movement, and subserve memory, emotion and cognition. Elucidating the properties of neural circuits requires an understanding of the cell types that comprise these circuits and their roles in processing and integrating information. However, since the description of diverse neuronal cell types over a century ago by Ramon y Cajal, we have barely scratched the surface of understanding the diversity of cell types in the brain and how each individual cell type contributes to nervous system function. Current approaches for classifying neurons rely upon features including the differential expression of small numbers of genes, cell morphology, anatomical location, physiology, and connectivity - important descriptive properties that nonetheless are insufficient to fully describe or predict the vast number of different cell types that comprise the mammalian brain. Here we propose a suite of technologies for identifying and classifying the myriad cell types present in the brain. Our method will be developed using layer 5 pyramidal cells from mouse somatosensory cortex as a model system. First, we will exploit the latest developments in DNA sequencing technologies to characterize gene expression profiles on single layer 5 neurons at high throughput. This information will be used to classify individual cells based on their transcriptome fingerprints. Second, genes found to define newly discovered neuronal subtypes will be used to gain genetic access to these cells using Cas9/CRISPR-mediated genome engineering to create transgenic reporter lines. Development of this technology promises to open a pipeline for the rapid generation of multigenic mouse reporter strains in which specific neuronal subtypes are uniquely labeled by combinations of tagged genes. Third, we will use these genetically engineered mice to confirm that our taxonomy represents distinct functional properties of the classified neurons. Our approach can ultimately be scaled up to generate a complete census of cell types in the brain, a critically needed resource for dissecting nervous system function with modern investigative tools.